1. Field of the Invention
The present invention relates to a fluorescence detector to be used as a detector in, for example, a liquid chromatograph, and more particularly, a fluorescence detector having a temperature control device for controlling the temperature of a sample to be introduced into a flow cell, and a liquid chromatograph having such a fluorescence detector.
2. Description of the Related Art
A liquid chromatograph has a detector for detecting each of the components of a sample separated by an analysis column, and an example of such a detector includes a fluorescence detector. In a fluorescence detector, a sample to be analyzed is introduced into a transparent container called ‘flow cell’, the flow cell is irradiated with light having a specific wavelength as excitation light, and the amount of fluorescence emitted from the excited sample is measured by a photodetector to determine the concentration or amount of each of the components of the sample.
However, many samples to be analyzed by a liquid chromatograph show a strong dependence on temperature of the amount of emitted fluorescence, and therefore in a case when fluorescence emitted from such a sample is detected under conditions where the temperature of the sample varies, there has been a problem that the amount of fluorescence detected by a photodetector varies with changes in the temperature of the sample, thereby making it impossible to obtain a good detection result.
In order to solve such a problem, a fluorescence detector having a heat exchanging portion provided upstream of a flow cell on a flow passage for flowing a sample and a temperature control system constituted from a Peltier device provided in the vicinity of the heat exchanging portion has been proposed (see, for example, Japanese Patent Application Laid-open No. 2000-346805), and this proposal has been carried out. This fluorescence detector is designed to allow heat exchange to be carried out between a sample passing through the heat exchanging portion and the temperature control system to control the temperature of the sample to be introduced into the flow cell by cooling.
Meanwhile, the sensitivity of a photodetector provided for detecting fluorescence emitted from a sample is also dependent on temperature. However, an error resulting from the temperature dependence of sensitivity of the photodetector is less than that resulting from the temperature dependence of the amount of fluorescence emitted from a sample, and therefore, conventionally, a higher priority has been given to keeping a sample to be introduced into a flow cell at a constant temperature.
However, some photoelectron multipliers as representative examples of photodetectors show a high rate of change of sensitivity with a temperature of 0.5%/° C. or higher. When such a photodetector whose sensitivity is strongly dependent on temperature is used, there is a case where the amount of fluorescence measured has a large error. Therefore, it has been necessary to select a photodetector whose sensitivity was weakly dependent on temperature to achieve higher sensitivity and highly-reproducible analytical results not dependent on its ambient temperature. However, this limited the choice of usable photodetectors, thereby causing an increase in the production cost of a detector.
Further, in a case where a photoelectron multiplier was used as a photodetector, there has been also a problem that the amount of dark current flowing through a multiplier tube was increased as the temperature of a detector was increased, thereby increasing noise.